1. Technical Field
The present invention relates to a light-emitting device, an image printing apparatus, and a manufacturing method of a sealing member.
2. Related Art
There are known exposure devices which are used as line-type optical heads of electro-photographic image printing apparatuses and which produce an electrostatic latent image on an image carrier (for example, photo-sensitive drum). JP-A-2004-195676 and JP-A-2004-195677 disclose exposure devices having a structure in which a light-emitting plate, on which a plurality of organic electro luminescent elements (organic light-emitting diodes (OLED)) are arranged, and a lens array, in which a plurality of lenses is arranged, are combined with each other. In the exposure device having such a structure, a plurality of ball lenses having a substantial spherical shape overlaps a plurality of organic electro luminescent (EL) elements, respectively, and light beams emitted from the corresponding organic EL elements are converged by the ball lenses. The converged light reaches an image carrier and thus an electrostatic latent image corresponding to the light is formed.
In FIG. 5 of JP-A-2000-158705, a different type of lens array is disclosed. In this lens array, plane-convex lenses are formed on both surfaces of a transparent plate member, and the plane-convex lenses on the front surface and the plane-convex lenses on the back surface form biconvex lenses. A plurality of biconvex lenses is disposed to overlap a plurality of organic EL elements, respectively, and light beams emitted from the corresponding organic EL elements are converged by the biconvex lenses. The converged light reaches an image carrier and forms an electrostatic latent image.
In order to sufficiently enhance performance of an exposure device employing the light-emitting device and the lens array, a large amount of light emitted from the organic EL elements must be introduced into the lenses disposed in a manner of overlapping the corresponding organic EL elements so that light use efficiency must be high. Accordingly, a distance between the organic EL elements and the lenses must be sufficiently small.
The light-emitting device used as the exposure device of the electro-photographic image printing apparatus is not a bottom emission type image printing apparatus in which light emitted from a light source emerges from the element substrate side, but a top emission type image printing apparatus in which light emerges from the opposite side of the element substrate. In the top emission type light-emitting device, light emitted from the organic EL elements emerges from the sealing layer side in which the sealing layer covers the organic EL elements in order to protect the organic EL elements from external air. It is preferable that the sealing layer is thick from the standpoint of improving reliability of the exposure device (for example, prolong lifespan of the organic EL elements). However, if the sealing layer is sufficiently thick, a distance between the lens array and the organic EL element array becomes undesirably larger. Accordingly, according to the known arts, the decrease of the distance between the lens array and the organic EL element array and the increase of reliability were in the tread-off relationship.